Comparative Planetology

The Outer Planets vs. the Inner Planets

The term "outer planets" refers to planets that orbit the Sun beyond the Asteroid Belt, including Jupiter, Saturn, Uranus, Neptune and Pluto. The inner planets are Mercury, Venus, Earth and Mars.

There are small, rocky inner planets (Mercury, Venus, Earth and Mars-sometimes called the terrestrial planets); giant, gas outer planets (Jupiter, Saturn, Uranus and Neptune, also called jovian planets); and dwarf planets, such as Pluto and Ceres. Pluto and its moon Charon have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material (such as frozen water, carbon dioxide, molecular nitrogen, methane and carbon monoxide).

Pluto and Charon are widely considered to be among the largest objects in the Kuiper Belt, a vast reservoir of icy objects located just outside of Neptune's orbit and extending out to about 50 AU from the Sun. The Kuiper Belt is thought to be the source of most short-period comets - those with orbits shorter than 200 years - so scientists really want to compare the composition and surface properties of Pluto and Charon to those of cometary nuclei.

Objects of the Outer Solar System

This mass vs. distance plot of outer solar system objects shows the enormous range in sizes of these bodies, from Jupiter (318 times the mass of the Earth) to comets (less than 1 billionth of Earth's mass). Pluto, Charon and Neptune's moon Triton fall in the 1/100 to 1/1000 Earth-mass range. Notice that some Kuiper Belt Objects (such as 1992 QB1, 1993 FW, 1993 HA2) appear to be much smaller than planets but much larger than comets.

Terrestrial Planets, Gas Giants and Dwarf Planets

Why are there only three types of planets? Why not a class for medium-sized planets?

The reason relies on two basic facts of solar system formation. In the large cloud of gas from which the solar system formed, 1) the temperature decreased farther away from the Sun (just as it feels cooler when you move away from a campfire) and 2) the amounts of different elements varied according to a well-defined distribution (the cosmic abundance of elements).

The most abundant element in the universe (by far) is hydrogen; the next-most abundant chemically reactive (not inert) element is oxygen. Combine them and you get H2O - water.

As the disk-shaped cloud of gas around the young Sun began to cool, relatively small amounts of metals and rocks began to condense. Farther out, beyond the "snow line" where temperatures were cooler, water ice condensed into "snow flakes." These flakes bumped into each other, sometimes sticking to make snowballs. As the snowballs bumped into each other and grew, a few snowballs had accumulated enough mass that their gravity could hold on to hydrogen, the lightest as well as the most abundant element. Very rapidly the bigger snowballs became giant planets.

Inside the "snow line" (thought to be at about 3-5 AU) it was too warm for the abundant water to condense; it remained as a vapor. Only the metals and rocks condensed - and since these elements are rare in the cosmic abundance, there was not much material to condense. Hence, in the inner solar system we only have small, rocky planets. In the outermost solar system, beyond the giant planets, not only did water freeze but so did other abundant molecules (such as carbon dioxide, methane and nitrogen). But there was just not enough "stuff" and/or time to accumulate objects larger than dwarf planets in the outer solar system.

A "Satellite System"

The satellites, or moons, orbit their parent planets. When there is more than one moon, one can refer to a "satellite system," the term used for all of the gas giant planets. Within a satellite system, the inclinations are often used to distinguish between moons that formed at the same time as the planet, creating something like a "miniature solar system" with the satellites orbiting in a plane nearly coincident with the equator of the planet, and moons that were captured later and whose orbital planes are highly inclined with respect to the planet's equator.

The Jupiter system as a great example. Click here to learn more about Galilean satellites as regular satellites.

Surface Processes on an Dwarf Planet

One process is sublimation and condensation. Pluto has three ices on its surface that also form its atmosphere: nitrogen, methane, and carbon monoxide. When the surface warms up, these ices can sublime into the atmosphere. When the surface gets cooler, the atmosphere condenses on the surface like frost on a window pane. Another process is photochemistry, or chemistry driven by photons (light). Bluer photons have more energy than red ones, and ultraviolet photons have enough energy to cause sunburns here on Earth, and to break chemical bonds in the frozen methane on Pluto's surface. When these bonds reform, they generally make molecules that are larger, darker and redder. Ice may also crack - making it brighter - or harden into large sheets, making it more transparent, like "black ice."

How Can Ice Behave Like Rock?

Ice can be much stronger than one might think. In general, however, the resistance of solid materials to mechanical failure (brittle fracture and faulting, or slow viscous creep) depends on: the strength of the chemical bonds that hold the solid's constituent atoms and molecules together; temperature; and, to a degree, the molecular weight of the atoms and molecules.

The hydrogen bonds that link one water ice molecule with another are reasonably strong, but nowhere near as strong as the covalent and ionic bonds between the silicon, oxygen and metal atoms (much as magnesium) in rock. Increasing temperature disrupts these chemical bonds, and conversely, decreasing temperature increases strength. Thus, in the outer solar system, where temperatures can be quite low, ice can be much stronger than the ice in our refrigerators or that which flows down mountainsides as glaciers. It is a rigid, hard material, and can be thought of as the "bedrock" of Pluto and Charon's geology, while weaker, more volatile ices (like nitrogen and methane) can play the roles of condensable solid or soft, deformable surface material.

Strictly speaking, though, cold water ice never achieves the bearing strength of cold basalt or granite, but on a subtler level, when calibrated against a material's elastic rigidity (resistance to shear deformation), water ice is one of the strongest materials known.

Geological Processes on Planetary Objects

The four main geological processes are impacts, volcanism, tectonics (cracking) and erosion.

Impact craters: In the solar system's formative years, when there was a lot of material left over from making planets, collisions were frequent and violent impacts frequently bombarded the surfaces of planetary objects. Craters produced by these impacts are readily visible on Earth's Moon and other planetary objects. Observations of the Moon and dating of lunar rocks the Apollo astronauts brought back to Earth tells us the frequency of impacts seems to have dropped dramatically about 3.8 billion years ago. Measuring the density of craters on the surface of a planetary object allows scientists to estimate the surface's age and when it was "resurfaced."

Volcanism: When the interior of a planetary object is heated (e.g., by the natural decay of radioactive elements or by tidal heating) then material underneath the surface melts and sometimes erupts onto the surface. On the terrestrial planets we are familiar with volcanos erupting molten rock as lava. On objects whose outer layers are ice rather than rock it can be water or other liquids that flow onto the surface, or sometimes gases that vent. The volcanism of these low temperature ices is called cryo-volcanism.

Tectonics: Any kind of cracking of a brittle surface layer that happens when the crust of a planet is stressed. Sometimes the stresses are caused by shrinking or expanding of the surface layer as the object cools down or heats up. Sometimes the cracks are associated with volcanic activity. Earth is the only planet that currently exhibits an extreme form of tectonics - plate tectonics - where the whole crust of the Earth is turned over and reworked every few hundred million years.

On a planetary scale erosion is a minor process but it can produce dramatic features (such as the Grand Canyon). The most effective agent of erosion is liquid water, though wind and ice can also cause erosion.

Temperature Controls

The temperature of a planet is mainly controlled by how much energy it gets from the Sun, which depends on the albedo (reflectivity) of the surface and its distance from the Sun. A planet gets more energy if it is closer to the Sun and reflects less sunlight back into space. To balance this, the planet also radiates energy in the infrared. The hotter the planet, the more energy it radiates. At some temperature, the incoming energy balances the outgoing energy. This is the "effective temperature." For a body without an atmosphere, like the Moon, this is the surface temperature.

Pluto has both reflective (bright) and absorbing (dark) patches. In general, scientists expect the the dark areas to be hotter than the bright ones (like walking on blacktop on the way to the beach -- ouch!). But on Pluto, frozen nitrogen sublimes from sunlit spots and recondenses on shadowy spots. It takes a lot of energy to sublime ice - this is why, for example, it takes lots of stove fuel to melt snow for your soup on a winter camping trip. Evaporation also takes energy, so you get chilly going out with wet hair. On Pluto, the transport of ice from one part of the planet to another also transports energy, tending to keep frost at a single temperature. If the atmosphere absorbs and radiates in the infrared, then the effective temperature corresponds to the temperature somewhere in the atmosphere, and the surface can be quite a bit hotter. On Earth, the average surface temperature is about 60° Fahrenheit (15° Celsius), but the effective temperature is only minus-4° Fahrenheit (minus-20° Celsius). This is called the greenhouse effect.

Pluto's Age

Pluto probably formed around the same time as the Sun and the other planets. The best estimate for the age of the solar system, and thus the age of Pluto, comes from radiometric dating of meteorites found on Earth. Analysis of radioactive isotopes in these meteorites all give the same answer: that the solar system is approximately 4.6 billion years old. Although there was presumably some variation in the formation times of the planets, circumstantial evidence suggests that the smaller objects all formed within a million years or so of the Sun. Thus, our best estimate for Pluto's age is 4.6 billion years.

Origin of Charon

Planets end up with moons, or satellites, by several different mechanisms. Charon is so large compared to Pluto, and orbits so closely, that the leading theory for its formation is the same theory thought to account for the Earth's Moon — a giant impact. There is so much angular momentum in the orbits of Charon and Pluto and in the spin of Pluto, that Pluto and Charon could never have been part of a single, stable body. In other words, Pluto and Charon had to begin as independent worlds, though not necessarily the same ones we see today!

A relatively slow, off-center collision by two comparably sized protoplanets would have resulted in the disruption of both and the merger of most of the debris into a rapidly rotating Pluto, but about 10% of the total mass would remain in close orbit and reaccrete (coalesce) into what we now know as Charon. Tidal friction would then slow the spins of both Pluto and Charon down and expand the orbital distance between the two.

That's the theory, anyway. Proving the impact part of such a scenario on a computer is difficult, and a matter of current research. The differences between Pluto and Charon, in density and composition, are essential clues to how (and indeed, if) giant impacts work, so the data returned by New Horizons will be crucial to understanding how both Pluto and Earth got their moons.